Electric motors are used inter alia in automation technology for servo motors, stepper motors or linear drives. Electric motors may be configured as rotary motors or configured as linear motors. In the case of a rotary motor, a rotating mover or rotor is driven by electromagnetic interaction with a fixed stator. In the case of a linear motor, the electromagnetic interaction required for the drive occurs between a stator and a mover which is movable along a path.
In the case of permanently excited electric motors, the drive force is generated by electromagnetic interaction between drive magnets and drive coils, through which electrical current flows, of the motor. Depending on the type of construction of the electric motor, it is possible either for the stator of the motor to comprise the drive coils and the mover to comprise the drive magnets, or else for the stator to comprise the drive magnets and the mover to comprise the drive coils.
As drive magnets, use is normally made of permanent magnets in a magnet arrangement composed of multiple drive magnets. Here, adjacent drive magnets of the magnet arrangement have opposite magnetization, such that an alternating magnetic field is generated along the magnet arrangement. The drive coils are normally wound around pole teeth which have a material of high magnetic permeability, generally iron, and which increase the magnetic flux density.
In the case of electric motors with permanent excitation, disruptive cogging torques, in particular cogging torques, arise, which are caused by magnetic interaction of the drive magnets with the pole teeth. The cogging torques can be reduced inter alia by means of an adaptation of the geometries of the drive magnets and of the pole teeth, in particular by means of a suitable adaptation of the number and dimensions of the drive magnets and pole teeth, and by means of a suitable adaptation of the pole coverage factor of the drive magnets.
In designing an electric motor, it is sought to achieve not only a reduction of the cogging torques but also as great as possible and as uniform as possible an introduction of force into the mover, the highest possible dynamics, and as high a power density as possible. The drive force that can be exerted on the mover of the electric motor is normally limited by the maximum magnetic field strength that can be generated by the drive magnets at the location of the drive coils.
To increase the magnetic field strength of the drive magnets at the location of the drive coils, a return plate composed of a ferromagnetic material is often arranged on a side of the magnet arrangement averted from the drive coils. By means of the return plate, the return of the magnetic field lines of the drive magnets on their coil-averted side is improved, and the flux density on the coil-facing side is increased. A magnet arrangement with return plate is described inter alia in the document DE 10 2011 075 445 A1. In the case of an electric motor in which the drive magnets are arranged on the movable mover, a return plate increases the moving mass of the motor, which has an adverse effect on its dynamics.
The magnetic field generated at the drive coils may also be increased by virtue of the drive magnets being arranged in a magnet arrangement configured as a Halbach arrangement. In the case of a Halbach arrangement, between in each case two drive magnets of opposite polarity, there is arranged a compensation magnet, the magnetic field of which intensifies the magnetic field of the drive magnets on the coil-facing side of the magnet arrangement and compensates said magnetic field on the coil-averted side of the magnet arrangement. Document U.S. Pat. No. 8,863,669 B2 describes a use of a magnet arrangement configured as a Halbach arrangement in an electric linear motor. A disadvantage of such a magnet arrangement is that high cogging torques arise here.
The document U.S. Pat. No. 7,538,469 B2 has disclosed an electric linear motor with a magnet arrangement configured as a modified Halbach arrangement. In the case of the modified Halbach arrangement, the compensation magnets of the magnet arrangement have in each case a width which is half that of the drive magnets. This reduces the cogging torques, but leads to only incomplete compensation of the magnetic field on the coil-averted side. Therefore, the magnet arrangement described in document U.S. Pat. No. 7,538,469 B2 comprises a return plate in order to reduce the magnetic resistance for the magnetic field emerging from the magnet arrangement on the coil-averted side.